1. Technical Field
The present invention relates to a temperature detector of a semiconductor circuit and, more particularly, to a temperature detector providing multiple detected temperature points using a single branch and a method of detecting a shifted temperature.
2. Description of the Related Art
One operating characteristic of a semiconductor device is its temperature characteristic. In the case of a CMOS device, the access time tACCESS increases as temperature is increased (A) (and thus the operating speed of the device decreases), and current consumption IDD increases as the temperature is decreased (B), as shown in FIG. 1. The temperature characteristic is important to devices that require a refresh operation, such as a dynamic random access memory (DRAM). The DRAM is a volatile memory and requires a refresh operation. In a DRAM cell, leakage current is increased when the temperature is increased and thus the data retention characteristic is deteriorated and the data refresh period tST is reduced.
Developments in electronics technology enable the design and cost-effective manufacturing of portable electronic devices, including pagers, cellular phones, audio players, calculators, lap-top computers, PDAs and so on. The portable electronic devices generally need DC power and thus at least one battery is used as an energy source for providing the DC power. In a battery-operated system, such as a portable electronic device, it is critically important to reduce power consumption. Particularly, in a sleep mode for saving power, circuit components included in the system are turned off. However, any DRAM included in the system should refresh DRAM cell data in order to continuously preserve the data.
One technique to reduce power required for a DRAM to operate is to vary the refresh period in response to temperature. Specifically, a temperature range is divided into multiple regions and the refresh period is increased in a low temperature region, that is, a refresh clock frequency is decreased, so as to reduce power consumption. Accordingly, a temperature detector is required to detect an internal temperature of the DRAM.
FIGS. 2A-B illustrate a conventional temperature detector. Referring to FIGS. 2A-B, a temperature detector 100 using a band gap reference circuit includes a plurality of branches 110, 120 and 130, PMOS transistors, and NMOS transistors. The temperature detector 100 further includes comparators 210, 220 and 230, which respectively compare temperatures OT1 through OTn, detected by the multiple branches 110, 120 and 130, with a reference temperature ORef.
The temperature detector 100 provides detected temperature points set to multiple specific temperatures. For instance, the first branch 110 may provide a detection point (or trip point) of 45° C. while the third branch 130 provides a detection point (or trip point) of 85° C.
The temperature detector 100 is very sensitive to variations in the semiconductor device manufacturing process. Thus, a temperature tuning operation for tuning a changed detection temperature point to a designed detection temperature point should be carried out for each DRAM chip at the wafer level. To perform temperature trimming during the temperature tuning operation, an operation of detecting a shifted temperature due to a variation in the manufacturing process must be carried out in advance.
FIG. 3 illustrates the distribution of shifted temperature detected from each chip in a lot, or batch, of chips. Referring to FIG. 3, when a refresh period, which is set when the temperature of a DRAM chip is in the range of 45° C. through 85° C., is 64 ms (X=64 ms), for example, the refresh period of the DRAM chip is set to half of 64 ms when the temperature is higher than 85° C. and three times 64 ms when the temperature is lower than 45° C.
However, when the temperature detector 100 employs the multiple branches 110, 120 and 130, the trip point of 45° C. of the first branch 110 may be shifted to a maximum of 50° C. and the trip point of 85° C. of the third branch 130 may be shifted to a minimum of 70° C. after the DRAM chip is manufactured. To tune the trip points, shifted to 50° C. and 70° C., to the desired set trip points, a separate trimming operation should be carried out for each branch. Accordingly, the temperature detector 100 requires a long period of time to detect a shifted temperature for each branch and to perform a trimming operation for the detected shifted temperature. Furthermore, the refresh period is varied by more than a factor of six, from three times the set refresh period to half the set refresh period in the range of 50° C. through 70° C.
Accordingly, it would be desirable to provide a temperature detector providing multiple trip points, or detection temperature points, using a single branch.
It would also be desirable to provide a method of detecting a shifted temperature using the temperature detector.
According to one aspect of the present invention, there is provided a temperature detector detecting a temperature shifted from a set target temperature, comprising an automatic pulse generator sequentially generating temperature control signals in response to a temperature detection signal; a comparator comparing detected temperatures with a predetermined reference temperature in response to the temperature control signals; a trip temperature increasing part comprising first short-circuiting switching transistors that selectively short-circuit a plurality of serially connected first binary weighted resistors in response to first test input signals and increasing the detected temperature when the shifted temperature is lower than the target temperature, the trip temperature increasing part being connected to a single branch; a trip temperature decreasing part comprising second short-circuiting switching transistors that selectively short-circuit a plurality of serially connected second binary weighted resistors in response to second test input signals and decreasing the detected temperature when the shifted temperature is higher than the target temperature, the trip temperature decreasing part being connected to the single branch; and a temperature detection controller selectively short-circuiting a plurality of serially connected resistors using third switching transistors in response to the temperature control signals to provide the detected temperatures, the temperature detection controller being connected to the single branch.
According to another aspect of the present invention, there is provided a method of detecting a shifted temperature that is changed from a set, target temperature, comprising sequentially generating temperature control signals in response to a temperature detection signal; selectively short-circuiting a plurality of serially connected resistors using switching transistors in response to the temperature control signals to provide detected temperatures, the switching transistors are being connected to a single branch; and comparing the detected temperatures with a predetermined reference temperature in response to the temperature control signals to search the shifted temperature.
Preferably, the shifted temperature detecting method further comprises increasing the detected temperatures using short-circuiting switching transistors that selectively short-circuit a plurality of serially connected binary weighted resistors in response to first test input signals when the shifted temperature is lower than the target temperature, and carrying out a trimming operation of short-circuiting the binary weighted resistors in response to the first test input signals obtained by binary weighted approximation. The short-circuiting switching transistors are connected to the single branch.
The shifted temperature detecting method further comprises decreasing the detected temperatures using short-circuiting switching transistors that selectively short-circuit a plurality of serially connected binary weighted resistors in response to second test input signals when the shifted temperature is higher than the target temperature, and carrying out a trimming operation of short-circuiting the binary weighted resistors in response to the second test input signals obtained by binary weighted approximation. The short-circuiting switching transistors being connected to the single branch.
According to the temperature detector of the present invention, the temperature detection controller connected to the single branch provides multiple trip point temperatures in response to the temperature control signals sequentially generated by the automatic pulse generator. Since a shifted temperature for the single branch is found and a trimming operation in response to the shifted temperature is carried out, test time is reduced. Furthermore, various refresh periods can be set in response to various trip point temperatures and thus consumption power of a DRAM can be decreased. The temperature detector of the present invention requires a layout area smaller than the layout area of the conventional temperature detector using multiple branches because the temperature detector of the present invention uses a single branch. Moreover, temperatures are shifted in the same direction such that 85° C. is shifted to 90° C. when 45° C. is shifted to 50° C. according to the temperature detector of the present invention. Thus, a stable refresh period is maintained even if the temperature is shifted.